1. Field of the Invention
The present invention relates to a wireless communication system, and more particularly, to an apparatus and method for Minimum Mean Square Error (MMSE) channel estimation that considers offset compensation in the wireless communication system.
2. Description of the Related Art
In the 4th Generation (4G) communication system, research is being conducted to provide users with services of various Qualities of Service (QoS) at a data rate of about 100 Megabits per second (Mbps). The typical 4 G communication system is an Institute of Electrical and Electronics Engineers (IEEE) 802.16 system. The IEEE 802.16 system employs an Orthogonal Frequency Division Multiplexing/Orthogonal Frequency Division Multiple Access (OFDM/OFDMA) scheme in order to support a broadband transmission network in a physical channel.
An OFDM scheme is a scheme of transmitting data using a multi-carrier. That is, the OFDM scheme is a type of Multi Carrier Modulation (MCM) scheme of converting serial input symbol sequences into parallel symbol sequences, modulating each of the symbol sequences into a plurality of subcarriers having mutual orthogonality, i.e., a plurality of sub-channels, and transmitting the modulated subcarriers.
The OFDM scheme based on multi-carrier transmission is advantageous as compared to other systems in estimating a channel having a multipath fading characteristic, because data signals are represented in a 2-dimensional structure of a symbol axis and a subcarrier axis and pilot signals also are transmitted in this structure. The number of assignable pilots is restricted according to the system requirements because, when a pilot signal is a bigger part of a signal, the channel estimation performance can be better, however, the transfer rate of a data signal is worse, relatively. After an OFDM system first estimates a channel of a pilot signal, the OFDM system estimates a channel of a data signal using a channel estimation value of the pilot signal. An example of a pilot signal channel estimation scheme is a Least Square (LS) scheme. Examples of data signal channel estimation schemes are linear interpolation, Low-Pass Filter (LPF), MMSE schemes and the like.
The linear interpolation scheme of estimating a channel between a Base Station (BS) and a Mobile Station (MS) is advantageous in that implementation is not overly complex, but is disadvantageous in that estimation performance for a channel having a multipath fading characteristic is degraded due to frequency selectivity and time variance.
On the other hand, the MMSE scheme has excellent estimation performance for a channel having a multipath fading characteristic. The MMSE scheme is known as a channel estimation scheme having good linear channel estimation performance. The MMSE scheme is a scheme of, after estimating a channel for a pilot tone in a Least Square (LS) technique, performing operation of auto-correlation between pilot tones and cross-correlation between a data tone and the pilot tone, and estimating a channel for the data tone as in Equation (1) below.
                                          h            ^                    d                =                                                            R                dp                            ⁡                              (                                                      R                    PP                                    +                                                            1                                              CINR                        P                                                              ⁢                    I                                                  )                                                    -              1                                ⁢                                    h              ^                        P            LS                                              (        1        )            
In Equation (1), “ĥd” represents a data channel estimation value, “Rdp” represents a cross-correlation value between a data tone and a pilot tone, and “RPP” represents an auto-correlation value between pilot tones. The “CINR” represents an estimated Carrier to Interference-and-Noise Ratio (CINR), and “ĥpLS” represents a pilot LS estimation value. Here, it is widely known that, on the general assumption that a complex gain of each path has independent identical distribution (i.d.d.) and has the same time-varying characteristic, a correlation function (R) can have a separation property of a form of Equation (2) below.
                              R          ⁡                      (                          correlation              ⁢                                                          ⁢              matrix                        )                          =                                            R              f                        ×                          R              t                                =                                    ∑              n                        ⁢                                          (                                                      σ                    n                    2                                    ·                                      ⅇ                                                                  -                        j                                            ⁢                                                                                          ⁢                      2                      ⁢                      π                      ⁢                                                                                          ⁢                      k                      ⁢                                                                                          ⁢                      Δ                      ⁢                                                                                          ⁢                      f                      ⁢                                                                                          ⁢                                              τ                        n                                                                                            )                            ·                                                J                  0                                ⁡                                  (                                      2                    ⁢                                          π                      ·                      l                      ·                                              T                        s                                            ·                                              f                        D                                                                              )                                                                                        (        2        )            
In Equation (2), the “Rf” represents a frequency correlation value, and the “Rt” represents a time correlation value. “n” represents the number of paths, and “σn2” represents an average power of an Nth multipath component. “k” represents relative subcarrier spacing, and the “l” represents relative symbol spacing. “Δf” represents subcarrier spacing, and “τn” represents a delay time of the Nth multipath component. “J0” represents a 0th Bessel function or a function performing function similar to this, and Ts” represents a symbol period, and “fD” represents maximum Doppler.
As above, the MMSE scheme has a characteristic in which the number of operations necessary for estimating a channel is considerably large compared to other schemes because of the operation of auto-correlation between pilot tones and cross-correlation between a data tone and a pilot tone and the like. Moreover, the MMSE scheme has a problem in that it is difficult to apply in a real system because of the need for prior knowledge of a power delay profile of a channel, a CINR, a speed and the like. Also, because the existing MMSE scheme does not consider frequency offset and time offset compensation of removing a phase bias element in a subcarrier/symbol direction, the MMSE scheme has a problem in that channel estimation errors caused by frequency selectivity and the Doppler effect are accumulated.